Finding Dark Galaxies

One of the great mysteries of our universe is whether there are dark matter galaxies, devoid of stars, haunting the universe. From the COSMOS survey, we know that dark matter and visible matter are not always located in the same place. This implies that there may be galaxies out there made entirely out of dark matter. The question is: how can we prove they do or don’t exist? Proving they completely don’t, never ever, no where in the visible universe exist is actually impossible. No matter how hard we look, it will always be possible for one dark galaxy to be hiding. On the other hand, proving the do exist, is tedious but possible. This is because even dark galaxies exert a gravitational pull on the stuff around them.

In a recent survey, lead by Igor Karachentsev of the Special Astrophysical Observatory in Russia*, a team of astronomers looked for the signature of dark galaxies in the light of isolated luminous galaxies. Here’s how it works: When two galaxies interact gravitationally the become extremely distorted. A quick look a few nearby colliding galaxies, such as “The Mice,”(1, top left) “The Tadpole,”(2, right) and the “The Antenna”(3, bottom left) all show signs of violent, shape altering, interactions. In each of these situations, we are looking at two visible galaxies interacting. If dark galaxies are out there, we should at some point find a lone luminous galaxy that looks like it has been tele-ported out of one of these train-wrecked galaxies and deposited in the middle of empty space. The distortions will be caused by the effects of the dark galaxy. (Images at left: 1&2. NASA, H. Ford (JHU), G. Illingworth (UCSC/LO), M.Clampin (STScI), G. Hartig (STScI), the ACS Science Team, and ESA, 3. Bob and Bill Twardy/Adam Block/NOAO/AURA/NSF)

Karachentsev and his team observed 1500 isolated galaxies and only found 8 systems that appeared significantly disturbed. Using the old 6-meter telescope in the Northern Caucuses, they carefully verify none of the small faint galaxies that appeared in the same field as the disturbed galaxy were located at the same distance. It appears that these systems are legitimate candidates for visible galaxy – dark galaxy collisions. This is both good and sad.

It is good because we can say that it is likely that dark matter galaxies are likely out there, living their non-luminous lives in the midst of their more illuminated brethren. That is just really cool. It is sad because when you look at 1500 places to potentially discover an interaction and only find 8 interactions, it implies these things just aren’t that common. So, as always seems to be the case, the really cool stuff is really hard to find and rare.

But, would the cool and interesting stuff be so cool and interesting if it was common?

Additional Random Notes:

* The Special Astrophysical Observatory in Russia is the first professional observatory I ever worked at. It was really cool to see some one from there publish a paper I wanted to write about 🙂

1) Neither of my computer’s keyboards have the old fashioned division symbol. You know, the one with the line and a dot above and below it that appears on every calculator keyboard. I remember this key being on my old type writer in high school. I’m not sure when it disappeared, and I’m very confused.

2) A quick look over my reading history shows that if a journal article contains the word “Ammonia” in the title, I’m pretty much guaranteed to never read it. I don’t know why.

ASCII does not have such a glyph. So, why would you have a keyboard version? One could have argued that instead of a 127 entry code, ascii should have used a 255 code, with more glyphs. But then the daisy wheels would have to be bigger, and twice as slow. Dot matrix came later, and laser printers later still.